Micro-truss structures (micro-trusses) are ordered three-dimensional (3D) open-cell frameworks typically formed by straight, elongated interconnected polymer strut-like structural elements. Micro-truss structures can be used as is, or used as templates to form ordered 3D micro-truss structures with other materials, such as metals or ceramics. Micro-truss structures are considered promising mechanical meta-materials for a number of applications such as in lightweight structural materials for building aircraft, 3D-routed optical cross connects, medical stents, batteries with volume structured electrodes, fuel cells, catalytic converters, and acoustic volume absorbers.
Several serial and parallel micro-truss fabrication methods have been proposed. Serial production methods include serial additive deposition processes, e.g., using inkjet-like systems (i.e., similar to “3D printing”), and serial self-writing methods that use 2-photon lithography (e.g., using Nanoscribe tools) to form the desired micro-truss topology inside a monomer volume. Although serial methods facilitate the generation of highly complex micro-truss topologies, these methods are too slow to create large volumes of micro-truss structures at a practical cost. In contrast, parallel self-writing methods facilitate the generation of micro-truss structures in high volumes at relatively low cost, but at the expense of some generality. Conventional parallel micro-truss fabrication methods typically involve a self-writing polymerizing process in which multiple truss elements are generated simultaneously (i.e., in parallel) to facilitate generating large area micro-truss structures on a commercially feasible scale. Because the present invention builds on conventional parallel self-writing micro-truss fabrication methods, both the general concept of self-writing polymerization and the properties and limitations of conventional parallel self-writing micro-truss fabrication methods are summarized in the following paragraphs.
Self-writing polymerization is a well-known effect in uncured materials (e.g., negative photoresists) that react to light exposure by undergoing polymerization in a way that produces both increased structural strength and an increase in refractive index. The increased refractive index reaction facilitates the generation of self-propagating lightguides (i.e., cylindrical polymer structures with slightly higher index than its surroundings) through the use of collimated light beams. That is, as a cylindrical region of the uncured material is polymerized by a collimated light beam, it forms a cylindrical polymer lightguide structure having a lower refractive index than the surrounding uncured material, whereby the cylindrical polymer lightguide structure serves to self-propagate (self-write) its own lengthwise growth by guiding the light beam deeper into the uncured material. Results show that this self-writing method is controllable tightly enough to allow for reasonably long, straight lightguides that, when directed in a manner that causes two lightguides to intersect without deleterious effects (i.e., a substantially integral interlocking joint is produced by the polymerized material forming the intersection, and the second lightguide is able to continue in a substantially straight line after the intersection).
Conventional parallel self-writing micro-truss fabrication methods takes advantage of the self-writing method to simultaneously generate a large number of parallel lightguide (truss) elements, for example, by directing collimated light through a reservoir containing a uncured using a mask with multiple apertures (openings), whereby elongated portions of the uncured exposed to collimated light beams passing through the apertures are polymerized to form the parallel micro-truss structural elements. After generating parallel micro-truss structural elements that extend in a first direction, the reservoir (or light source) is rotated, and the collimated light is again passed through the mask apertures to form a second set of parallel micro-truss structural elements extending in a second direction. A 3D open-cell micro-truss framework is thus formed by directing the collimated light at four or more predetermined angles through the apertures, whereby the generated structural elements extend at predetermined angles and fuse together at intersections. The excess uncured is then developed and drained from the reservoir, the mask is removed, and the micro-truss structure is cleaned and subjected to further processing (e.g., a thermal post-cure process that leaves the micro-truss stiff enough to be self-supporting, and removal from its supporting substrate).
While the above-described conventional parallel self-writing micro-truss fabrication method yields impressive results, it is limited to straight lightguide members at limited angles (e.g., 30 to 40 degrees from the surface normal) due to refraction, aperture shrinking and other issues that occur at oblique angles. Specifically, it is not possible to fabricate micro-truss structures having in-plane structural (lightguide) elements, which are very important for the mechanical properties of micro-truss frameworks, using the conventional parallel self-writing method. Consequently, the conventional parallel self-writing micro-truss fabrication method cannot be used to generate superior micro-truss structures, such as those having the “Octet Truss” topology, which was identified by Buckminster Fuller and others as an ideal space frame unit.
What is needed is an enhanced self-writing method that overcomes the limitations of the conventional methodologies to allow for the low-cost, large-scale fabrication of micro-truss structures having horizontal (in-plane) structural elements. In particular, what is needed is an enhanced self-writing method that facilitates the formation of in-plane (i.e., horizontally-oriented) polymer lightguides while retaining the (fast) self-writing characteristics associated with the conventional parallel micro-truss fabrication methodology, and does not introduce alignment requirements that preclude scalability to large area micro-truss structures.